When a user photographs an object by holding a camera by hand, the image of the object may blur due to camera shake when the shutter speed is slow. Techniques for correcting such blur due to camera shake by image processing have been studied. In particular, techniques for correcting blur due to camera shake by using a plurality of images acquired by photographing an object at various exposure times, in order to reduce the number of images to use for image correction, have been proposed (see, for example, Japanese Laid-Open Patent Publication No. 2007-324770, Japanese Laid-Open Patent Publication No. 2009-284001 and Japanese Laid-Open Patent Publication No. 2011-44846).
For example, Japanese Laid-Open Patent Publication No. 2007-324770 discloses an imaging device for acquiring images without blur and reduced noise, by a smaller number of images. The imaging device captures a plurality of images, including an image which has a short exposure time and high resolution but which contains a lot of noise, and an image which has a long exposure time but which contains little noise and has low resolution. Then, the imaging device separates edge blur and noise from a differential image between two images, based on a threshold value, and synthesizes the two images by changing the synthesis ratio between the images. At the time of synthesis, with respect to pixels that are determined to be edges, the imaging device increases the synthesis ratio of the image which has a short exposure time and high resolution but which contains a lot of noise.
Furthermore, Japanese Laid-Open Patent Publication No. 2009-284001 discloses an image processing device that generates a synthetic image by synthesizing the first image, a second image which is acquired by photographing under a shorter exposure time than the first image and low sensitivity, and a third image in which the high frequency components of the first image are filtered. The image processing device calculates the edge intensity values on a per pixel basis, increases the synthesis ratio of the first image with respect to pixels having greater edge intensity values, and increases the synthesis ratio of the second image with respect to pixels where the difference between the second image and the third image is less.
Furthermore, Japanese Laid-Open Patent Publication No. 2011-44846 discloses an image processing device that synthesizes a short-exposure image of a short exposure time and an adequate exposure image of a long exposure time. The image processing device divides the short-exposure image into a plurality of partial image areas, and increases the synthesis ratio of the short-exposure image to the adequate exposure image, when the edge intensity value with respect to the pixel position of interest is greater and the area edge amount for the partial image area where the pixel position of interest belongs is greater.
With any of the above techniques, for the edge areas where the object's edges are captured, the synthesis ratio of an image of a short exposure time increase compared to an image of a long exposure time. Consequently, even in a synthesized image, the noise components to be contained in the edge areas increase relatively. Depending on cases, due to the difference between the amount of noise components contained in the edge areas and the amount of noise components contained in areas around the edge areas, the granularity of the edge areas becomes distinct, and, as a result, a synthetic image may give an impression of having poor overall quality to the user.